I am thinking of buying some hobby servos. Servos are so costly in India like 20$. I dont understand how much torque or speed or which type of gears like metal or plastic i require. Basic uses will be to turn sensors like ping))). What if i need to use it as wheels for a robot or on a robotic arm, which servo will you recommend then? If you are in India and came across some cheap ones, please tell me. Or some item with free shipping on ebay!

For applications like a scan servo on which you're going to mount an ultrasonic sensor, ebay is your friend! You get those tiny 9 gram servos (with free shipping often) for around $2-$3. They're good enough for the job, but can be tricky to mount things on because they're fairly small.

As for wheels, do you really think servos are good for the job? They can be slightly painful to modify for continuous rotation, and are VERY slow, not to mention expensive ($20 for a decent servo seems about right to me). Why not use those geared dc motors, they are quite cheap, and will do the job just fine! Controlling them with an Arduino is also easier, but you need a motor driver such as the L298N (good tutorial: http://www.youtube.com/watch?v=UF70yDi8BRI). The driver is about $5, and can control 2 motors, so usually you just need one for a robot with differential drive!

I had a look at one of the 9g servos, and it can only move 180º (at least the one I bought could). Modifying the thing is going to be HARD though, because it is so small, and trying to file of the bits of the gear and so forth is near impossible...